The technique of isotope separation, specifically uranium enrichment, using isotopically selective photoexcitation and ionization, is, for example, shown in U.S. Pat. Nos. 3,772,519, 3,939,354, or U.S. application Ser. No. 368,298 filed June 8, 1973, all commonly assigned herewith, or in German publication No. 2,312,194. It is known that isotopically selective photoexcitation can be achieved from several low-lying energy levels, such as the ground and 620 cm.sup.-1 levels using appropriately tuned laser radiation.
An excited state produced by the photoexcitation from the plural low-lying energy levels will typically be subject to self-lasing of the excited medium either by reflection of radiation from the boundaries of the excited medium or through sufficient amplification length of the medium. It is additionally desired that each excitation step, particularly those involving isotopic selectivity, as is the case for the excitation from the low-lying energy levels to the first excited state, have a large cross-section, that is be strongly coupled to the excited state. Plural excitation transitions to a single excited level are not, however, all likely to possess such a desired, high cross-section.
Due to theoretical particle distributions among all excited levels, the first excited level is likely to possess a high population contrasted with other levels. A single first excited state is thus likely to be strongly coupled to other, less populated energy states to which a population inversion may readily exist, thereby enhancing the potential for self-lasing decay from this single excited level.
The numbers of excited states in the typical region for a first excited state, for example below one-half of the ionization potential, is far more restricted than the numbers of states above that level in the uranium atom. This further limits the selection of states which possess the desired properties described above without any of the problems mentioned.